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SubjectRe: implementing soft-updates
On Tue, Apr 09, 2002 at 08:41:28PM -0400, Albert D. Cahalan wrote:
...
> While ext2 fsck doesn't guarantee anything, in practice it is far
> more reliable than ufs fsck. If you change the algorithms to be
> like those used by BSD, then you may lose some of the ability to
> recover. Remember, fsck isn't just for power failures. It tries
> to piece together a filesystem that has suffered disk corruption
> caused by attackers, kernel bugs, fdisk screwups, MS-DOS writing
> past the end of a partition, Windows NT Disk Manager, viruses,
> disk head crashes, and every other cause you can imagine. If you
> change fsck to make BSD-style assumptions about write ordering,
> you weaken the ability to deal with disasters.

I disagree. In fact the current BSD softupdate code guarantees that all
that ever happens is that freed blocks are not entered into the free
block list. Something fsck can fix in background on a life system. See
M. Kirk McKusicks BSDcon 02 paper 'Running fsck in background.'

Your argument that faulty hardware may create havoc with your on-disk
data structures is something every file system is prone to unless it
uses a raw-read-after-write for checking purposes. Something which
definitely kills disk performance.

The background fsck capability, just like journalling or logging, are
typically only in needed in 24/7 systems (sure, they are nice to have in
your home system, but do you _REALLY_ need them? i don't!) and those
system typically are run on proven hardware which is operated well
within the specs. So please don't construct these kinds of arguments.

The fact that the BSD FFS in it's currently released version (which does
not include snapshot and background fsck capability) is considered to be
one of the more reliable file systems around, even when softupdates are
enabled, speaks for itself. So please just as you don't want horror
stories about Linux ext2 spread: don't do it yourself.

Alexis, if you're looking for a rewarding Linux project, don't focus too
much on softupdates, the majority of linux users/developers couldn't
care less. I wonder sometimes if this is perhaps because BSD did it
first?

Read M. Kirk McKusick's paper on fsck and snapshots (it's in the
proceedings of this years BSDcon, available from Usenix) and try to
implement the snapshot capability for ext2/ext3. Everyone of us who has
to do live backups of production systems will thank you if you get that
development started. I found that Mr. McKusick is somebody who is very
helpful towards people trying to understand his work, so you might get
help from him if you get stuck. OTOH if you avoid the buzzword
'softupdates' many Linux file system hackers will be much more inclined
to help you out with the Linux part.

Yours,
Dominik Kubla
--
"Those who would give up essential Liberty to purchase a little
temporary Safety deserve neither Liberty nor Safety." (Benjamin Franklin)
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